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Investment Securities (Net Realized Investment Gains (Losses)) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | $ (10.2) | $ 7.4 | $ (24.9) | $ 21.0
Trading securities, Tax on Realized Holding Gain (Loss) on Investments | 2.3 | (0.6) | 11.2 | (3.6)
Trading securities realized holding gain (loss) on investments after tax | (7.9) | 6.8 | (13.7) | 17.4
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | (0.6) | 11.0 | (0.2)
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax | 0.0 | 0.0 | 0.0 | 0.0
Trading securities, realized foreign currency gain (loss) net of tax | 0.0 | (0.6) | 11.0 | (0.2)
Net realized investment (losses) gains, pre-tax | (10.2) | 6.8 | (13.9) | 20.8
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities | 2.3 | (0.6) | 11.2 | (3.6)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax | (7.9) | 6.2 | (2.7) | 17.2
Fixed maturity investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | (5.1) | 0.6 | (27.6) | (0.4)
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | 1.2 | 18.2 | 2.7
Net realized investment (losses) gains, pre-tax | (5.1) | 1.8 | (9.4) | 2.3
Short-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | 0.0 | (0.1) | (0.8) | (0.1)
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | 0.0 | 0.0 | 0.0
Net realized investment (losses) gains, pre-tax | 0.0 | (0.1) | (0.8) | (0.1)
Common equity securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | (1.5) | 4.9 | 10.5 | 18.5
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | 5.4 | 0.0 | 6.0
Net realized investment (losses) gains, pre-tax | (1.5) | 10.3 | 10.5 | 24.5
Other long-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Trading Securities, Realized Gain (Loss) | (3.6) | 2.0 | (7.0) | 3.0
Foreign Currency Transaction Gain (Loss), Realized | 0.0 | (7.2) | (7.2) | (8.9)
Net realized investment (losses) gains, pre-tax | $ (3.6) | $ (5.2) | $ (14.2) | $ (5.9)

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